An increasing number of applications and systems for communicating information nowadays makes use of the spread spectrum technique. The spread spectrum technique is a digital modulation technique in which a digital signal is spread over a wide frequency band so that it has a noise-like spectrum. This can be done by breaking up or “chopping” each data bit of the digital signal into multiple sub-bits (commonly called chips) that are then modulated and up-converted to a carrier frequency. The chopping may be done by multiplying the digital information signal with a so-called pseudo-random code or PN code. By using orthogonal codes for different communication links, the same frequency band can be used for different simultaneous communication links.
Using the same PN code as the transmitter, a receiver can correlate the received, spread signal and reconstruct the data signal while other receivers that use other codes or other transmission techniques cannot. One of the advantages of using the spread spectrum communication technique is the robustness to narrow band interference signals. Because spread-spectrum receivers are rapidly being introduced in applications and systems meant for the consumer markets, the cost of the receiver system is a major determining factor in order to remain competitive.
A specific class of spread spectrum systems are devices and receivers for position determination. Such devices are gaining importance for both the consumer market and for high precision applications. Most of the existing systems are based on the American Global Positioning System (GPS) system. Because this is also a military system, precise position determination can be made difficult by the satellite operator deliberately introducing errors (called “anti-spoofing”). Moreover, in many areas the number of visible satellites can be too limited to determine an accurate position. These two problems can be reduced by also using a second positioning system such as the Russian Global Orbiting Navigation Satellite System (GLONASS) system.
Combined GPS and GLONASS receivers have been reported, e.g. S. Riley, N. Howard, E. Aardoom, R. Daly, and P. Silvestrin, in “A combined GPS/GLONASS high precision receiver for space applications”, ION-GPS 95, Palm Springs, USA, September 1995, or Japanese patent application JP 7128423-950519, “Receiver Common to GPS and GLONASS”.
A GPS receiver is known from U.S. Pat. No. 5,293,170 which can be used with P-code modulated signals which have been modulated with an unknown code. Implementations of this known device require a lot of registers.
U.S. Pat. No. 5,600,670 describes a GPS receiver which includes a hierarchical chain of channel modules which includes slave modules and a master module. The known system does not provide sufficient flexibility.